Previously, thermoacoustic apparatuses have been known as apparatuses for conducting energy conversion between thermal energy and acoustic energy. For example, apparatuses shown in Patent Document 1 and Patent Document 2, as described below, have been known.
A thermoacoustic apparatus shown in this Patent Document 1 will be described. As shown in FIG. 17, this thermoacoustic apparatus is provided with a loop tube 200 having a hollow portion in the inside, an acoustic wave generator 300 for generating an acoustic wave through self excitation in the loop tube 200, and an acoustic heat exchanger 400 for converting the acoustic energy to the thermal energy. These acoustic wave generator 300 and the acoustic heat exchanger 400 are composed of a stack 303 sandwiched with a pair of metal heat exchangers 301 and 302 and a stack 403 sandwiched with a pair of metal heat exchangers 401 and 402 and are attached in the loop tube 200 individually. These heat exchangers 301, 302, 401, and 402 have a plurality of holes, grids, or the like for passing the acoustic wave through the inside and are configured in such a way that heat can be input from or output to the outside of the loop tube 200. Among these heat exchangers, the upper heat exchanger 301 on the acoustic wave generator 300 side is set at, for example, 700° C. to 800° C. by inputting factory waste heat, automobile waste heat, or the like from the outside. The lower heat exchanger 302 and the upper heat exchanger 401 on the acoustic heat exchanger 400 side are set at relatively low temperatures. For example, the temperature is set at about 18° C. to 20° C. by circulating water in the surroundings. On the other hand, the stacks 303 and 403 disposed in the acoustic wave generator 300 and the acoustic heat exchanger 400 are formed from ceramic, a resin, a metal, or the like and have configurations in which a plurality of transmission paths having very small diameters are disposed along an axis direction of the loop tube 200. When heat is applied to the heat exchanger 301 of the thermoacoustic apparatus having the above-described configuration, an acoustic wave with a plurality of wavelengths is generated through self excitation after a while, and a stable standing wave and a traveling wave are generated in the loop tube 200 after a lapse of a certain time. The acoustic energy due to the standing wave and the traveling wave is transferred along the loop tube 200 to the acoustic heat exchanger 400 side and expands or shrinks a working fluid in the stack 403 there. The thermal energy released from or absorbed by the working fluid because of the expansion or shrinkage is transferred along a wall surface in the stack in a direction reverse to a transfer direction of the acoustic energy. The heat of the heat exchanger 402 is thereby drawn out so that the heat exchanger 402 is cooled. The heat resulting from cooling is output to the outside so as to cool an object to be cooled.
Furthermore, regarding such a thermoacoustic apparatus, an apparatus for improving the energy conversion efficiency has also been proposed. For example, in Patent Document 2 described below, as shown in FIG. 18, a thermoacoustic apparatus having a narrow portion 10, in which the inner diameter of a loop tube is made relatively smaller than those of the other portions is proposed. In FIG. 18, reference numeral 20 denotes an acoustic wave generator and reference numeral 30 denotes an acoustic heat exchanger in which a temperature gradient is generated between heat exchangers because of the acoustic wave output from the acoustic wave generator 20. As described above, in the case where the narrow portion 10 is disposed in the loop tube, an acoustic wave stream and a mass stream generated in the loop tube 200 can be reduced to some extent. Consequently, transfer of heat in the loop tube is reduced and the energy conversion efficiency can be improved.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-274100    [Patent Document 2] Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-535597